Transcription
All right, so this, uh, this slide is looking again at how molecules cross the cell membrane. And in this particular video, we are going to focus on the molecules that cannot easily cross the membrane. So they fall into two main groups.
So the first group is shown over here, and these are what we call ions. Now, we should know what ions are from unit one. Um, ions are these small particles that have a charge. So, for example, potassium ion, a chloride ion, hydrogen ion, things like sodium ions as well. Any of these are tiny. I mean, they're smaller than molecules. They're a single particle like an atom, but it has a charge. And even though they're very tiny and you think they might be able to sneak in between these phospholipids and get through the membrane, they can't. And the reason why is because this charge that they have, positive or negative, doesn't matter which one, um, the charge makes them so hydrophilic. They love water so much, they're attracted to water so much that they are, they just do not want to be in this region here, in the hydrophobic core of the membrane. And so these ions cannot get through here. They, so they, they basically kind of bounce off, like this is showing. If they're on the in outside, they can't get in. If they're on the inside, they can't get out because they cannot transverse or travel over this super hydrophobic area because they are so charged and so attracted to water. So that's ions. They cannot cross the cell membrane unless they have help. Now, of course, they can cross if they have a tunnel. And if you remember from a previous video, we talked about how protein molecules can create tunnels or channels, and that's how these ions move in and out of cells through tunnels, but they cannot cross the membrane without help.
The second kind of molecule shown over here, and these are large molecules with charges or partial charges. So these molecules are large to start with. So like a protein is a big molecule. Even glucose, I mean, it's a pretty big molecule, C6H12O6. And then any chains of glucose, they're just too big. Uh, and starch. Starch is a chained up glucose, a bunch of glucose is all chained up. And so these molecules are large. And so to start with, their size is against them in terms of getting through these little chinks that open up between the phospholipids. And, but the other thing that is working against them is that they usually have partial charges. So there's going to be some areas where there's going to be partial positives and some areas, they're going to be partial negatives. And those areas are going to be so attracted to water, um, that they're not going to want to leave the water behind. And they're definitely not going to want to go into this area here where it's very non-watery, where it's very hydrophobic. So large molecules, if they have charges or partial charges, are not going to cross. That's these. And tiny, tiny particles like ions aren't going to be out across either because of their charge. So those are things that cannot cross the cell membrane. And that's most things, honestly, in biology. Most stuff cannot cross the membrane without help. There's a very small subset of molecules that can cross, and that's these here that we recovered on a previous video.